In the wireless arts, desensitization of radio receivers caused by internal interference or noise sources is a problem in different radio technologies, such as for example global positioning system/global navigation satellite system (GPS/GNSS), digital video broadcast for handheld devices/China Mobile multimedia broadcasting (DVB-h/CMMB), broadcast FM (frequency modulation) particularly where the FM receiver has an internal antenna, and also more generally in cellular radio bands. Conventionally various filtering components have been used, as well as ferrites and parallel capacitors, to mitigate the interference. For example, one conventional arrangement for GPS is to dispose surface acoustic wave (SAW) filter at the output end of a transistor operating as a low noise amplifier (LNA). But disposing the bandpass SAW filter after the LNA is not effective in suppressing in-band intermodulation distortion (IMD) products which originate from the LNA itself.
Other sources of noise or interference is direct current DC voltage feed circuitry and a LNA collector circuit which generate internal interference signals that can easily leak to the LNA. In these instances the interference signal becomes mixed with the received signal and are subsequently amplified by the LNA, which causes degradation of the receiver sensitivity.
Mobile radio handsets also impose multiple other sources of noise/interference. In the audio system the audio power amplifier circuit and the internal hands-free IHF speaker can interfere with radio reception. In the display system the display circuitry itself, the user interface and the DC to DC converter for the display can interfere. In various video systems there may be interference from the TV output chip or from the camera. More generally, interference in a handset receiver may arise from Bluetooth or other near field communication radios, the engine hardware, the subscriber identity module (SIM) card, the micro SD (secure digital) plug and the DC plug. The FM receiver with an internal FM antenna/radiator and LNA, and also an internal mobile TV receiver, are particularly susceptible to electromagnetic interference (EMI) due to their relatively lower frequency signals, and for which the inventor is unaware of sufficiently effective solutions in the prior art.
The teachings below address the above EMI issues, but while presented in the context of mobile handset implementations the described circuitry can be employed in any number of electronic devices which employ an amplifier, whether low noise or power amplification.